Many types of conventional light sources are known, examples of which include, but are not limited to, incandescent light sources, fluorescent light source, lasers, gas discharge light sources, chemical light sources, and electroluminescent light sources. In electroluminescent light sources, materials emit light in response to an electric current or an electric field being applied thereto. Moreover, electroluminescent (EL) light sources represent a broad category of light sources, such as light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), polymer light-emitting diodes (PLEDs), which are a subset of OLEDs, light-emitting electrochemical cells (LECs), field-induced polymer electroluminescent lamps (FIPELs), thick film electroluminescent (TFEL) lamps, thin film electroluminescent lamps, and electroluminescent wires.
Thick film electroluminescent lamps, thin film electroluminescent lamps and electroluminescent wires typically include one or more layers of phosphor crystals that emit light in response to an electric field being applied thereto. The electric field being applied is usually an alternating electric field having an alternating frequency in order of 1 kHz. In simple terms, thick film electroluminescent lamps, thin film electroluminescent lamps and electroluminescent wires can be regarded as various forms of light-emitting capacitors (LECs). For light to be generated from a light-emitting capacitor (LEC), its electrodes must be connected in a manner so that the electric field can be applied to the phosphor crystals. The electric field is conveniently generated using an inverter, which is typically configured to deliver an alternating current (AC) sine wave excitation signal to the LEC; optionally, excitation with temporally abruptly-changing waveforms can be applied, for example, “square waves”. A voltage and a frequency of the applied excitation signal influence brightness and operating lifetime of the LEC, wherein up to a certain saturation point, higher voltage and higher frequency both result in higher LEC brightness, but reduced operating lifetime. Preferably, an AC sine wave signal is applied, wherein a gradual change of electric field associated therewith is less harsh on the phosphor crystals than abrupt changes occurring in an electric field resulting from applying a square wave signal to the phosphor crystals, as aforementioned.
As previously mentioned, LECs are often divided into three different categories: thick film electroluminescent (TFEL) lamps, thin film electroluminescent lamps and electroluminescent wires. TFEL lamps are typically characterized by layer thicknesses in a range of 1 μm to 100 μm, and are typically fabricated using wet printing techniques. Thin film electroluminescent lamps are typically characterized by layer thicknesses of less than 1 μm, and are typically fabricated using vacuum deposition techniques. Depending upon the intended application, TFEL lamps are typically operated at a frequency in a range of 400 Hz to 1500 Hz, and excited by a signal having route-mean-square (RMS) value in a range of 60 volts to 120 volts. Operation at higher frequency and/or voltage is possible, but with correspondingly shorter lifetime and diminishing gains in brightness.
Further, the LECs operate on fundamentally different operating principles as compared to other EL light sources. For example, OLED-based EL light sources allow electric current to flow therethrough, as they operate with bias; however, LECs operate based on charge/discharge cycles. Typically, LECs include a front electrode layer, a rear electrode layer, a dielectric insulating layer and a micro-encapsulated solid phosphor layer. In operation, when an alternating current is applied to the front and rear electrode layers, an electromagnetic field is created. This electromagnetic field in turn excites the phosphor layer which produces a luminous energy.
Some advantages and disadvantages of LEC lamps are provided in a table below.
Advantages of LEC LampsDisadvantages of LEC LampsLightweightLow luminance light source, luminance ofless than 300 candela per square meterThinLimited range of emission coloursFlexibleRequire a customized power supply, such asan inverterCool-to-touchEnergy inefficient light source, luminousefficacy of less than 10 lumens per wattDevoid of glass to breakDevoid of gas to escapeUniform light emissionover a large areaSimple to installSimple to maintainCompatible withroll-to-roll (R2R)manufacturing method
Electroluminescent light sources, in general, and more specifically, LEC lamps and TFEL lamps, can be routinely fabricated on lightweight, thin and flexible substrates, such as polycarbonate, polyethylene teraphthalate (PET) or polyethylene naphthalate (PEN), using vacuum deposition and/or wet-process deposition techniques. A typical commercial example is Light Tape, manufactured by Electro-LuminX Lighting Corporation. These flexible TFEL lamps can be flexed with one-dimensional (1D) curvature and wrapped around various objects. However, more complex shapes cannot be covered using conventional TFEL lamps. For many applications, it is desirable that a TFEL lamp has substantial curvature in two dimensions (2D). Examples of shapes with surfaces with 1D curvature are cylinders and cones. These shapes have surfaces with zero Gaussian curvature and are developable. In other words, these surfaces can be flattened out onto a plane without distortion. Examples of shapes with surfaces with 2D curvature are spheres, spheroids, partial spheroids, three-dimensional saddles and depressions. These shapes have surfaces with non-zero Gaussian curvature and are non-developable. In other words, these surfaces cannot be flattened out onto a plane without distortion.
From this description, it is clear that electroluminescent light sources prepared on planar substrates cannot be conformed to have 2D curvature without distortion of their plane. The problem is that distortion of a plane within an emissive region can cause critical damage to light-emitting layers and/or complimentary device layers, such that an electroluminescent light source may cease to function, or function with reduced efficiency and/or uniformity. The typical cause of this critical damage is cracking or delamination of the light-emitting layers and/or the complimentary device layers that can lead to open and/or short circuits and/or increased sheet-resistance within these layers
In particular, for electroluminescent light sources, such as OLEDs, PLEDs and LECs, transparent electrodes, which are most typically fabricated from Indium Tin Oxide (ITO), are well known to suffer from cracking. This is described in relation to TFEL lamps and problems caused by distortion in a published PCT patent application WO2001/010571A1, entitled “Printable Electroluminescent Lamps Having Efficient Material Usage and Simplified Manufacture Technique”.
In an attempt to circumvent this problem, various conventional techniques have been employed. One conventional technique, related to TFEL lamps, has been described in U.S. Pat. No. 6,054,809 to Bryan D. Haynes, et al., entitled “Electroluminescent Lamp Designs”. This conventional technique involves patterning emissive areas around regions of substantial 2D curvature, such that there are no substantial 2D curvature in the emissive areas, but only in-between the emissive areas. However, this conventional technique suffers from several disadvantages. Firstly, extra steps required for patterning the emissive area are complex and time-consuming. Secondly, these extra steps increase an overall cost of manufacturing. Thirdly, the conventional technique does not enable continuous regions of light emission across regions of 2D curvature, thereby limiting visual effect and range of applications.
Another conventional technique, related to OLED lamps, has been described in US patent application no. 20120161610, entitled “Light Extraction Block with Curved Surface”, and US patent application no. 20120162995, entitled “3D Light Extraction System with Uniform Emission Across”. This conventional technique involves use of additional optical components to create an appearance of 2D curvature of a region. In this conventional technique, a light extraction block, with an external surface with 2D curvature that optionally includes a light-scattering layer, is optically coupled to a planar OLED device. When operated, light generated in the planar OLED device propagates into the light extraction block, wherefrom the light is scattered by the external surface with 2D curvature, giving a visual effect of a continuous light source with 2D curvature. However, in this instance, one or more device layers, where photons are generated and from where the light is emitted initially, are planar, optionally with 1D curvature, but not with 2D curvature. The visual effect is created only through the use of the additional optical components, namely, the light extraction block with the light-scattering layer. However, this conventional technique suffers from several disadvantages. Firstly, the additional optical components are complex, and add bulk to the light source, thereby limiting range of applications. Secondly, the additional optical components are expensive.
Yet another conventional technique is described in U.S. Pat. No. 6,926,972 B2 to Michael Jakobi, et al., entitled “Method of Providing an Electroluminescent Coating System for a Vehicle and an Electroluminescent Coating System Thereof”. This conventional technique involves applying electroluminescent coatings, in particular, to automobiles, using spray-coating techniques. Although not described in the U.S. Pat. No. 6,926,972 B2, it is feasible that the technique provided therein could be applied to coating electroluminescent layers onto surfaces with substantial 2D curvature. However, it has yet to be demonstrated that this can be done uniformly with high precision required to achieve a desired uniformity of luminance and/or chromaticity. This is because a TFEL lamp acts as a light-emitting capacitor (LEC) in which a layer thickness variation of as little as 1 μm to 5 μm can lead to noticeable differences in light source luminance and/or chromaticity.
In light of the foregoing, there exists a need for a method of manufacturing an electroluminescent device that is operable to emit light in a substantially uniform manner with substantially uniform luminance and chromaticity.